Rapidly increasing use of fossil fuels has led to an increase in demand for use of alternative or clean energy. In light of such trends, generation and storage of electricity using electrochemical reaction are a very active area of research.
In recent years, representative examples of electrochemical devices using electrochemical energy are secondary batteries, and application range thereof continues to expand.
Recently, technological development and increased demand associated with portable equipment such as portable computers, cellular phones and cameras have brought about an increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and operating electric potential, long lifespan and low self-discharge have been actively researched and are commercially available and widely used.
In addition, increased interest in environmental issues has led to a great deal of research into electric vehicles, hybrid electric vehicles or the like as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles. These electric vehicles and hybrid electric vehicles generally use nickel-metal hydride secondary batteries as power sources. However, a great deal of study associated with lithium secondary batteries with high energy density and discharge voltage is currently underway and some are commercially available.
Conventional typical lithium secondary batteries use graphite as an anode active material. Lithium ions of a cathode are repeatedly intercalated into and de-intercalated from the anode to realize charge and discharge. The theoretical capacity of batteries may vary depending upon the type of the electrode active material, but generally cause deterioration in charge and discharge capacity in the course of the cycle life of the battery.
The primary reason behind such phenomenon is that separation between an electrode active material or separation between the electrode active material and a current collector due to volume variation of the electrode, as batteries are charged and discharged, results in insufficient realization of function of the active material. In addition, in the process of intercalation and de-intercalation, lithium ions intercalated into the anode cannot be sufficiently de-intercalated and active sites of the anode are thus decreased. For this reason, charge/discharge capacity and lifespan of batteries may decrease as the batteries are cycled.
In particular, in order to improve discharge capacity, in the case where natural graphite having a theoretical discharge capacity of 372 mAh/g is used in combination with a material such as silicon, tin or silicon-tin alloys having high discharge capacity, volume expansion of the material considerably increases, in the course of charging and discharging, thus causing isolation of the anode material from the electrode material. As a result, battery capacity disadvantageously rapidly decreases over repeated cycling.
Accordingly, there is an increasing demand in the art for binder and electrode materials which can prevent separation between the electrode active material, or between the electrode active material and the current collector upon fabrication of electrodes via strong adhesion and can control volume expansion of electrode active materials upon repeated charging/discharging via strong physical properties, thus improving structural stability of electrodes and thus performance of batteries.
Polyvinylidene difluoride (PVdF), a conventional solvent-based binder, does not satisfy these requirements. Recently, a method for preparing a binder, in which styrene-butadiene rubber (SBR) is polymerized in an aqueous system to produce emulsion particles and the emulsion particles are mixed with a neutralizing agent, or the like, is used and is commercially available. Such a binder is advantageous in that it is environmentally friendly and reduces use of the binder and thereby increasing battery capacity. However, this binder exhibits improved adhesion maintenance due to the elasticity of rubber, but has no great effect on adhesion force.
Accordingly, there is an increasing need for development of binders which improves cycle properties of batteries, contributes to structural stability of electrodes and exhibits superior adhesion strength.